|STIS Instrument Handbook for Cycle 25|
9.1 After you establish the set of scientific exposures and any additional target acquisition or calibration exposures that you require for your program, you are ready to determine the total number of orbits to request. Generally, this is a straightforward exercise involving compilation of the overheads on the individual exposures, packing the exposure plus overhead time into individual orbits, and tallying up the results to determine your total orbit request. In some cases, it may be an iterative process as you refine your exposure requests to more fully exploit the orbits.The Call for Proposals and the HST Primer provide information on the Observatory policies and practices with respect to orbit-time requests. Below, we provide a summary of the STIS-specific overheads and give several examples that illustrate how to calculate your orbit requirements for Phase I Proposals. Note that the HST Primer provides a more limited summary of overheads that are intended to be conservative. In the unlikely case that an overhead calculation using the HST Primer requires less time than one calculated using the information in this chapter, you should use the result from this chapter to ensure that you do not end up with a time allocation that is insufficient to carry out your program in Phase II.Alternatively, you could also use the APT to create a simple program. APT automatically takes the appropriate overheads into account and provides a statistically estimated orbit length to assist in constructing the observation program.